This invention relates generally to electromagnetic flowmeters, and more particularly to a bi-directional flowmeter capable of accurately measuring the flow rate of a fluid flowing in a line in either direction, the measurement being immune to changes in fluid conductivity, viscosity and density.
An electromagnetic flowmeter is a volumetric fluid flow rate device utilizing the characteristics of a metered conductive fluid to produce an induced voltage when flowing through a magnetic field. The operation of the meter is based on Faraday's Law of Induction which states that the voltage induced across a conductor as it moves at right angles through a magnetic field is proportional to the velocity thereof.
In the flowmeter, the fluid to be measured is conducted through a flow tube having a pair of electrodes mounted therein at diametrically-opposed points, a magnetic field being generated by an excitation winding in a plane perpendicular to the longitudinal axis of the tube, which plane includes the transverse axis common to the electrodes. If one considers a segment of the metered fluid as a conductor whose length D is equal to the diameter of the pipe, then as the fluid conductor moves at a velocity V through a magnetic field B, the voltage E induced across this conductor in the plane of the meter electrodes will be proportional to the rate of fluid flow.
This may be expressed mathematically by the following equation: EQU E=(I/C)BDV,
where C is a dimensionless constant.
By providing a magnetic field B of high strength, a favorable signal-to-noise ratio is obtainable in the output of the flowmeter. The reason for this will be evident from the foregoing equation, in that for a given fluid velocity V, an increase in the strength of the magnetic field B will give rise to an increase in the induced voltage E. The polarity of the flow signal will depend on flow direction, the polarity reversing when the direction of flow reverses. If the electromagnet is energized by an a-c source, the phase of the signal will change by 180.degree. C. during a reverse flow condition.
In the Schmoock U.S. Pat. Nos. 3,260,109 and 3,254,243, there are disclosed electromagnetic flowmeters whose electrode signals are applied to converters which include a feedback loop producing a frequency output proportional to flow rate, the feedback loop being intended to suppress spurious in-phase and quadrature components generated as a result of capacitive and inductive couplings between the magnet coil and the electrode loop.
An improved form of converter for an electromagnetic flowmeter is disclosed in the Mannherz et al. U.S. Pat. No. 3,783,687 whose entire disclosure is incorporated herein by reference.
As pointed out in the Mannherz et al. patent, the magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the d-c signal current flowing through the liquid acts to polarize the elecrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though a-c operation is clearly advantageous in that polarization is obviated and the a-c flow-induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublsome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and PA1 2. induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the a-c flow-induced voltage. Actual tests have indicated that this is not true, in that a component exists in-phase with the flow induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but elimination of its in-phase component has not been successful. Existing A-C operated electromagnet flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltages both with respect to phase and magnitude.
Hence, it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi./dt), serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change in the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition (d.phi./dt)=0 is satisfied. But d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered, as previously explained. Hence in the Mannherz et al. flowmeter, in order to obtain the positive benefits of a steady state field without the drawbacks which accompany continuous d-c operation, the steady state flux field is periodically reversed or interrupted.
In the Mannherz et al. flowmeter, the excitation current for the electromagnet in the flowmeter primary is a low-frequency "square wave equivalent" serving to produce a periodically-reversed steady state flux field whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects. Because switching transients are developed at the points of polarity reversal, the steady state condition of the flux field is disturbed at these points. A significant feature of the Mannherz et al. flowmeter is that flow-measurement is taken in the secondary associated with the flowmeter primary only in the intervals when the flux achieves a steady state condition, the unsteady state transients being effectively blocked out.
As pointed out in the copending patent application Ser. No. 745,862 of Suzuki, filed Nov. 29, 1976, entitled "Bi-Directional Output Electromagnetic Flowmeter" (now U.S. Pat. No. 4,089,219 ), a disadvantage of existing types of signal converters for electromagnetic flowmeters is that the converter is operable only when the fluid being metered flows in a given direction, for it does not respond when the input signal applied to the converter from the primary results from flow in the reverse direction.
In certain practical applications, the need exists for a magnetic flowmeter having bi-directional characteristics. Thus in service water systems in which the same supply pipe is used to receive and to deliver service water, one requires a bi-directional flowmeter to indicate flow rate in either direction.
To render a magnetic flowmeter bi-directional, Suzuki provides a polarity detector adapted to sense the direction of flow in the line. The detector operates in conjunction with an input signal inverter and a switch actuated by the detector output to select either the inverted or the non-inverted input signal so that the input signal applied to the converter is always in the same direction, and an output signal is yielded at all times regardless of the direction of flow.
Thus Suzuki renders a flowmeter bi-directional by reversing the sense of the input signal representing flow in the reverse direction whereby the input to the converter is always in the same polarity. The drawback to the Suzuki arrangement is that signal reversal at the converter input can give rise to the injection of unwanted offset voltages in the converter, in that the signal levels at this point are at the millivolt level where they may be substantially affected by microvolt offsets. These offsets are virtually impossible to remove even from a well designed printed circuit board.
Another problem encountered when using input signal reversal is that the outputs of the various amplification stages in the converter may contain d-c offset voltages inherent in the amplifiers themselves. This is especially true in the approach taken by Mannherz et al. where the low frequency signals must be decoupled between stages to preserve their integrity. When such a signal is reversed, both the a-c and d-c components are reversed and the resulting d-c will cause a unit step to propagate through the feedback loop, generating a substantial "bump" in the output which constitutes an erroneous flow reading.